Recently, in a monitoring field, there is a growing need for an image recognition technique for detecting a position and a size of an object from an image photographed with an imaging device. In order to realize such an image recognition technique, it is necessary to make a coordinate set on an image photographed with a camera (hereinafter referred to as an “image coordinate”) and a coordinate set on a real space (hereinafter referred to as a “world coordinate”) correspond to each other. A camera parameter is used for this correspondence.
Camera parameters are camera information that indicates, for example, a focal length or a direction of a camera and can be classified roughly into two, internal parameters and external parameters. Math. 1 is an internal parameter matrix, with f, a, s, and (vc, uc) indicating a focal length, an aspect ratio, a skew, and a central coordinate of an image coordinate, respectively.
                    K        =                  [                                                    f                                            sf                                                              v                  c                                                            0                                                                    0                                            af                                                              u                  c                                                            0                                                                    0                                            0                                            1                                            0                                              ]                                    [                  Math          .                                          ⁢          1                ]            
Math. 2 is an external parameter matrix, with (r11, r12, r13, r21, r22, r23, r31, r32, r33) and (tx, ty, tz) indicating directions of a camera and a world coordinate of a camera installation position, respectively.
                    D        =                  [                                                                      r                  11                                                                              r                  12                                                                              r                  13                                                                              t                  X                                                                                                      r                  21                                                                              r                  22                                                                              r                  23                                                                              t                  Y                                                                                                      r                  31                                                                              r                  32                                                                              r                  33                                                                              t                  Z                                                                                    0                                            0                                            0                                            1                                              ]                                    [                  Math          .                                          ⁢          2                ]            
When these two parameter matrices K and D and a constant λ are used, an image coordinate (u, v) and a world coordinate (XW, YW, ZW) are made correspond to each other by a relational expression of Math. 3.
                              λ          ⁡                      [                                                            u                                                                              v                                                                              1                                                      ]                          =                  KD          ⁡                      [                                                                                X                    w                                                                                                                    Y                    w                                                                                                                    Z                    w                                                                                                1                                                      ]                                              [                  Math          .                                          ⁢          3                ]            
When defined by Euler angles, (r11, r12, . . . r33) indicating the directions of a camera in the external parameters are expressed by three parameters, pan θ, tilt φ, and roll ψ that are installation angles of a camera. Therefore, the number of camera parameters necessary for correspondence between the image coordinate and the world coordinate is 11 derived by adding five internal parameters and six external parameters. In Math. 3, meanings of the camera parameter matrices remain unchanged even when multiplied by a constant. Therefore, λ and the parameter matrices K and D may be arranged into one matrix and expressed as Math. 4. Additionally, in Math. 4, when c34 is fixed at 1, the number of unknown parameters is 11. Obtaining these 11 parameters is synonymous with obtaining five internal parameters and six external parameters in Math. 3.
                              [                                                    u                                                                    v                                                                    1                                              ]                =                              [                                                                                c                    11                                                                                        c                    12                                                                                        c                    13                                                                                        c                    14                                                                                                                    c                    21                                                                                        c                    22                                                                                        c                    23                                                                                        c                    24                                                                                                                    c                    31                                                                                        c                    32                                                                                        c                    33                                                                                        c                    34                                                                        ]                    ⁡                      [                                                                                X                    w                                                                                                                    Y                    w                                                                                                                    Z                    w                                                                                                1                                                      ]                                              [                  Math          .                                          ⁢          4                ]            
Thus, Math. 3 can be eventually modified as Math. 5, and a matrix C in Math. 5 becomes a camera parameter obtained eventually. A calibration technique is needed to obtain this camera parameter C.
                              [                                                    u                                                                    v                                                                    1                                              ]                =                              C            ⁡                          [                                                                                          X                      w                                                                                                                                  Y                      w                                                                                                                                  Z                      w                                                                                                            1                                                              ]                                ⁢                      (                          C              =                              [                                                                                                    c                        11                                                                                                            c                        12                                                                                                            c                        13                                                                                                            c                        14                                                                                                                                                c                        21                                                                                                            c                        22                                                                                                            c                        23                                                                                                            c                        24                                                                                                                                                c                        31                                                                                                            c                        32                                                                                                            c                        33                                                                                    1                                                                      ]                                      )                                              [                  Math          .                                          ⁢          5                ]            
In general calibration, a camera parameter C is calculated by photographing a specific object with a target camera to input a plurality of combinations of a world coordinate of a feature point and an image coordinate corresponding to the feature point in Math. 5. For example, in PTL 1, this coordinate information is acquired manually.